Ascaris lumbricoides, parasites that inhabit the intestines of their hosts, possess proteins which react specifically with the digestive enzymes trypsin, chymotrypsin, pepsin and caroboxypeptidases A and B and inactivate them. Some of these Ascaris proteins (inhibitors of the digestive enzymes) are specific for the enzymes of their host and are very poor inhibitors of the enzymes of other species. Experiments are underway to: 1. Study the intracellular localization of these inhibitors in worms bathed in solutions of the host digestive enzymes to which a fluorescent label has been attached. These host enzymes possessing fluorescent labels are taken up by the worm and their location in the worm is being studied. Preliminary evidence indicates that the labeled host digestive enzymes are not degraded by the worm before their entry into the worm. 2. The site on the trypsin inhibitor from Ascaris which comes in contact with the host trypsin has been located and studies are being done to determine the influence of amino acid residues in this immediate area on the binding between inhibitor and host enzyme. Similar studies are being extended to the Ascaris protein inhibitors which inactivate both chymotrypsin and elastase. In addition the question of whether the same contact site on the inhibitor is used in interaction with chymotrypsin and elastase is being investigated. 3. The amino acid sequence of the chymotrypsin/elastase inhibitor is nearing completion. The 66 residues have been placed and 3 ambiguities and about 3 instances of amino acid exchanges are still being studied. BIBLIOGRAPHIC REFERENCES: 1. Homandberg, G.A. and Peanasky, R.J. "Interactions of Exopeptidases with Specific Proteins from Ascaris lumbricoides" in "Protides in Biological Fluids XXIII" (H Peeters, ed.) pp. 279-284, Pergamon Press, Oxford, England (1975). 2. Homandberg, G.A. and Peanasky, R.J., "Characterization of Proteins from Ascaris lumbricoides which Bind Specifically to Carboxypeptidase", J. Biol. Chem. 251, 2226-2223 (1976).